A method of making an austempered ductile iron article is disclosed, and more particularly, a method of making large austempered ductile iron articles, and even more particularly, a method of making an austempered ductile iron main shaft of a wind turbine.
Various alloys have been considered and used to produce large shafts, such as the main shaft of a wind turbine. As an example, forged steel alloys are commonly used for main shafts in wind turbine applications. While forged steel wind turbine shafts are useful, they are generally costly due to their size and the mechanical processing (e.g., forging), heat treating and machining required to obtain the necessary alloy properties.
Due to their strength, toughness, castability and machinability, ductile iron (cast nodular iron) alloys have also been used to produce wind turbine main shafts. Typical production methods for these shafts have included conventional sand casting, which has generally been employed to produce relatively smaller shafts having wall section thicknesses of less than fifteen centimeters. While this approach has been useful to produce ductile iron wind turbine shafts, the shafts are not well-suited to the demanding service requirements of more recent wind turbine designs, which include larger sizes, including higher casting weights, larger diameters and larger shaft wall section thicknesses. As larger main shafts become required for wind turbine applications, their design requirements for strength and toughness have exceeded the capability of conventional cast ductile iron alloys, and methods that enable the manufacture of ductile iron wind turbine main shafts having larger sizes, including higher casting weights, larger diameters and larger shaft wall section thicknesses are very desirable.
Therefore, improved methods of making ductile iron articles, particularly large ductile iron articles, such as the main shafts of wind turbines, are very desirable.